The invention relates generally to electrostatic sensors, and more specifically to capacitive micromachined ultrasound transducers (cMUTs).
Transducers are devices that transform input signals of one form into output signals of a different form. Commonly used transducers include, heat sensors, pressure sensors, light sensors, and acoustic sensors. An example of an acoustic sensor is an ultrasonic transducer, which may be implemented in medical imaging, non-destructive evaluation, and other applications.
One form of an ultrasonic transducer is a capacitive micromachined ultrasound transducer (cMUT). A cMUT cell generally includes a substrate that contains a lower electrode, a diaphragm suspended over the substrate by means of support posts, and a metallization layer that serves as an upper electrode. The lower electrode, diaphragm, and the upper electrode define a cavity. As will be appreciated by one skilled in the art, the support posts typically engage the edges of the diaphragm to form a cMUT cell. Further, a voltage applied between the lower electrode and the upper electrode causes the diaphragm to vibrate and emit sound, or in the alternative, received sound waves cause the diaphragm to vibrate and provide a change in capacitance. The diaphragm may be sealed to provide operation of the cMUT cells immersed in liquids.
As described above, a cMUT cell generally includes a diaphragm disposed over a vacuum cavity and the cavities in the cMUTs have been selectively etched through openings in the diaphragm to form the underlying cavity. Traditionally, these cMUTs are fabricated employing surface micromachining techniques. However, as will be appreciated, cMUTs fabricated employing surface micromachining techniques suffer from low yield and non-uniformities in the diaphragm. Alternatively, a silicon-on-insulator (SOI) wafer may be bonded to a silicon substrate that has cavities lithographically produced in an oxide cover layer. These bulk-micromachined cMUTs provide better predictability, reproducibility and uniformity of the diaphragms compared to the surface-micromachined cMUTs. However, use of the SOI wafers may not be cost effective. Furthermore, the process flexibility is limited by using SOI wafers and it is difficult to generate complex diaphragm structures using the conventional cMUT fabrication technology known in the art.
Therefore, in order to ensure predictability, reproducibility and uniformity of the diaphragms with low cost, high availability, and flexible design, it may be desirable to develop techniques that alleviate the problems associated with the current fabrication techniques employed to fabricate cMUT diaphragms.